A rotation detector has been disclosed in Patent Reference 1. In the rotation detector, a magnetic detection element, which is formed from a Hall IC or the like mounted on a circuit board, and a magnet are provided in a substantially cylindrical resin case, which has a detection surface for detecting an object to be detected, along the detection surface. In the rotation detector, it is arranged that the circuit board, the magnetism detection element and the magnet are sealed with a seal member formed of epoxy and such. In the case of using such a rotation detector as a rotation detector for a two-wheeled vehicle (a motorcycle, for example), the rotation detector is generally mounted to a mission case, a sprocket cover or the like of the vehicle to detect a mission gear, a sprocket or the like, which rotates inside the mission case, the sprocket cover or the like, as an object to be detected.
In the case that such a rotation detector is used for detecting the mission gear, the sprocket or the like as an object to be detected, the object to be detected has more or less clearance. This causes vibration of the object to be detected due to body rigidity, exhaust gas volume or the like of a vehicle (the two-wheeled vehicle), so that a gap in position of detection between the object to be detected and the rotation detector varies. This causes a change in magnetism of the rotation detector to cause a detection noise. Accordingly, there is a problem that the noise causes an error in operation of an indicator for displaying a speed, for example, in accordance to output data outputted from the rotation detector. In view of the problem, the applicant has proposed a rotation detector disclosed in Patent Reference 2.
The rotation detector is to be mounted to a front wheel (a wheel) of a two-wheeled vehicle such as a motorcycle. The rotation detector comprises a housing having an insertion section in which an axle is inserted, the axle to which the front wheel is mounted, and has plural magnetic poles, which rotate together with the front wheel, in the insertion section. An object to be detected, which is formed from a magnetic medium such as a plastic magnet, is mounted to the rotation detector. The rotation detector is arranged so that a magnetism detecting means formed from a Hall IC and such would detect a change of the magnetic pole due to rotation of the object to be detected.
The object to be detected is formed from a cylindrical part having the magnetic pole and a rotation transmitting piece for transmitting rotation of the front wheel to the cylindrical part, the cylindrical part and the rotation transmitting piece being formed into one body from a plastic magnet and such. The rotation transmitting piece is led out from the cylindrical part on an opening part side of the housing outward to be formed so as to correspond to a concave-shaped mounting part provided in a hub of the front wheel. The rotation transmitting piece is provided in plural places of a circumferential edge of the cylindrical part.
The rotation detector having such a structure is provided so as to be held between the front wheel and a front fork. That is to say, the rotation detector is provided so as to be held between (fastened to) the front wheel and a front fork by inserting in the insertion section the axle to which the front wheel is mounted, by fitting the rotation transmitting piece into the mounting part provided in the hub of the front wheel and by fixing the axle projecting toward the outer side of the housing from the insertion section with a nut member through the front fork.    Patent Reference 1: JP 2-264817 A    Patent Reference 2: JP 9-229714 A